Any investigation of biological markers in psychiatric illness must assess the effects of variables that reflect the physiologic state of the organism. We have carefully assessed an in vitro laboratory measurement of sodium-dependent lithium efflux and demonstrated requisite precision, reproducibility, and mood-state independence. The finding that lithium efflux is increased in essential hypertension emphasizes the need to fully understand the effects of physiologic variables on this measurement before it can be applied to diagnostic studies. Our group has observed the effects of hypertension, blood pressure, body mass, sex, and race on lithium efflux values in an affectively ill population. While we have compared the distribution of values in affectively ill patients with controls, there have been no studies looking at a large group of non-psychiatric patients carefully screened for a wide range of medical variables. The goal of the two year study is therefore to assess the physiologic factors affecting sodium-lithium transport measurements in a population determined to be free of psychiatric illness. Primary factors of interest are age, sex, race, blood pressure, hypertension, body mass, and alcohol intake. Variables of secondary interest are family history of hypertension, family history of psychiatric illness, differences between pre-and post-menopausal females, tobacco intake, major medical illness, and medication. Four hundred volunteers will be recruited from Portes Cancer Prevention Center, a private health screening facility associated with Northwestern University, and will be selected for absence of major psychiatric illness using the Diagnostic Interview Schedule (DIS). The importance of family psychiatric history will be assessed with the Research Diagnostic Criteria for Family History (RDC-FH). Blood samples will be taken for lithium efflux determinations. Our hypothesis maintains that physiologic factors may be found in persons without psychiatric history to influence lithium transport measurements in a predictable way. This study offers a thorough assessment of physical characteristics affecting lithium transport, which must be the necessary next step for the application of this measurement to large scale studies of biological markers in psychiatric illness.